As a photoelectric conversion device using an amorphous semiconductor film as a photo active layer, a solar cell, an image input sensor (image sensor) and an photo sensor have been practically used. Particularly, a solar cell using an amorphous silicon film has been known as a power source of a desk calculator, and has been widely used.
An amorphous silicon film has characteristic features, in comparison to a crystalline silicon material, in that a film having a large area can be produced at a low temperature of 400.degree. C. or lower, and the thickness that is sufficient to absorb light as a photoelectric conversion layer is as thin as about 1 .mu.m. Therefore, saving of the silicon resource and the energy required for its production can be expected, and it has attracting attention as a material of low cost in comparison to the conventional materials.
A diode structure having a pin junction has been generally used in the field of a solar cell, an image sensor and a photo sensor, for improving photoelectric conversion efficiency and photo response property. While p type, i type and n type layers all can be formed with amorphous silicon films, it has been known that in order to obtain good photoelectric conversion characteristics, the p type and n type semiconductor layers are formed with microcrystalline silicon films. This is because as the photoelectric conversion is mainly performed in the i type layer by light absorption, the p type and n type layers preferably have high light transmission property and is preferably formed with a material having high conductivity to attain good contact with an electrode. A microcrystalline silicon film has low light absorption loss and high conductivity, and is suitable as the material for the p type and n type layers.
An amorphous silicon film is produced by a chemical accumulation process using glow discharge plasma under reduced pressure (plasma CVD process). A plasma CVD apparatus, which is composed of a reaction chamber, an evacuation means for maintaining the reaction chamber under reduced pressure, a gas introducing means for introducing a raw material gas, a means for generating glow discharge plasma in the reaction chamber, and a means for holding and heating a substrate, is used. While silane (SiH.sub.4) is generally used as the raw material gas, disilane (Si.sub.2 H.sub.6) gas can also be used. The raw material gases may be after diluting with hydrogen (H.sub.2) gas.
A microcrystalline silicon film is produced by using a mixed gas of SiH.sub.4 gas and H.sub.2 gas, as a raw material gas, under the conditions in that a diluting ratio of the H.sub.2 gas is higher than the SiH.sub.4 gas. It has been known that a microcrystalline silicon film, to which no impurity element determining the conductive type, p type or n type, is added, exhibits n type conductivity. In general, an impurity gas containing an element determining the conductive type, p type or n type, is added on formation of the film to further improve the conductivity and to control the conductive type of p type or n type.
In the field of semiconductors, elements represented by boron (B), aluminum (Al), gallium (Ga) and indium (In), which belong to the IIIb group in the periodic table, have been known as an element determining p type conductivity, and phosphorous (P), arsenic (As) and antimony (Sb), which belong to the Vb group in the periodic table, have been known as an element determining n type conductivity. In the general plasma CVD process, an impurity gas represented by B.sub.2 H.sub.6 and PH.sub.3 is mixed with the raw material gas on the film formation. The addition amount of the impurity gas added is generally about from 0.1 to 5% relative to SiH.sub.4, and about 10% at most.
Because the microcrystalline silicon film and the amorphous silicon film have a low process temperature, an organic resin material can be used as a material for a substrate of a photoelectric conversion device, in addition to a glass material.
In the basic process for production of a solar cell and an image sensor formed on a substrate, a first electrode is formed on the substrate, a photoelectric conversion layer composed of a pin junction is formed on the first electrode in intimate contact therewith, and a second electrode is accumulated thereon. In the formation of the pin junction, the process is continued without breaking vacuum to improve the characteristics of the junction boundary.
At this time, it has been known that when the impurity gas is added to the raw material gas to form a p type or n type semiconductor film, a slight amount of the impurity gas and its reaction product adhere on the reaction chamber and a discharge electrode, which is a part of the means for generating glow discharge plasma. When an i type amorphous silicon film, which is substantially intrinsic, is continuously formed without addition of any impurity gas, there is a problem in that the residual impurity is released and incorporated into the i type amorphous silicon film. The substantially intrinsic i type silicon film is produced to have a defect density in the film of 1.times.10.sup.16 per cubic centimeter, and thus if the impurity element is incorporated in a concentration of from several tens to several hundreds ppm, it forms an impurity level to change the characteristics of the film.
In order cope with such a problem, a plasma CVD apparatus of separated multichamber type has been developed, in which plural reaction chambers are provided, which are separated from each other by a gate valve provided between the reaction chambers. Therefore, in order to produce the pin junction according to the conventional process, at least three reaction chambers for forming the p type, i type and n type semiconductor layers.
As a result, a gas introducing means for introducing SiH.sub.4, H.sub.2, B.sub.2 H.sub.6 and PH.sub.3, an evacuation means, and a glow discharge plasma generation means must be provided in each of the reaction chambers, and thus the constitution of the apparatus becomes complicated and exaggerated. The maintenance of the apparatus requires much labor, accordingly.
From the standpoint of the production process, after a semiconductor film of one conductive type is formed, the substrate must be moved from one reaction chamber to another reaction chamber, and the introduction and evacuation of the reaction gas must be conducted. These procedures must be repeated in order. Therefore, the reduction in time required to form the photoelectric conversion layer is naturally limited. Even when a technique of high-speed film formation is employed to reduce the time required for forming a film, the time required for transfer of the substrate and introduction and evacuation of the gas becomes a bar in reduction of the production time.
In the conventional technical field of a solar cell, it has been known that the concentration of a p type impurity in a p/i boundary of a photoelectric conversion layer is continuously changed to form a continuous boundary for improvement in junction characteristics. In order to realize such a technique in the conventional process, a gas containing a slight amount of a p type impurity element must be precisely controlled by using a computer.